Electrochemical sensors are used to determine the concentrations of various analytes in testing samples such as fluids and dissolved solid materials. A typical electrochemical sensor consists of three electrodes: a sensing electrode, a counter electrode, and a reference electrode. The accuracy of a particular sensor's measurements depends on the ability to measure the potential difference between the sensing electrode, whose potential varies with the analyte concentration in the measured sample analyte solution, and the reference electrode, which ideally maintains constant potential.
In typical, commercially available electroanalytical measurement systems, the physical interface between the reference electrode (typically the electrolyte of the reference electrode) and the sample solution is referred to as the liquid junction. The potential at the metal electrode within the liquid of the liquid junction is related to a number of factors; it is an object of every reference electrode design to minimize the effect of the factors that would cause the potential of the electrode to vary in any way over time. However, the liquid junction and the potential between the metal electrode and the outside environment are difficult to control and maintain at a constant level. Typically, it is the change in the electrode potential of the metal within the liquid junction that introduces error into the electrochemical measurement and causes the need for frequent sensor calibration.
One problem with junction structures is that they typically allow the sample analyte solution to enter the junction structure, which limits the useful lifetime of the reference electrode. This transport of analyte solution into the junction, whether by diffusion, migration, convection, or other means, results in the contamination of the junction structure and a resultant undesirable variation in the liquid junction potential. Such variation typically necessitates recalibration of the electroanalytical measurement system. If this type of contamination continues over time, the junction structure may become fouled or clogged and develop even larger offset potentials and/or potentials that chronically drift despite repeated attempts at recalibration. Such contamination takes away from the robustness of a reference electrode and prevents it from being used for long periods of time (i.e., long-lived). In addition, the analyte solution will often transport past the junction structure and reach the reference half-cell, possibly causing additional adverse reactions.
Another problem associated with the junction relates to the requirement that the activity of an ion or several ions or compounds in the electrolyte adjacent to the reference electrode must remain constant. The activity of such species must remain constant since they equilibrate with the electrode so as to fix the absolute potential at a constant value.
Accordingly, the stability of the reference electrode and hence, the accuracy of the potentiometric measurements, are dependent on the constancy of the liquid junction, the constancy of the ionic activity of an ion or compound that determines the potential of the electrode, and the constancy of the potential across the ionic liquid. If a highly stable and reliable reference electrode is not used for electrochemical measurements, the deviation in the potential leads directly to a measurement error. As such, the reference electrode is a basic and important element for the electrochemical sensor.
The use of reference electrodes is typically necessary in the field of microelectromechanical systems (MEMS). One subset of MEMS devices are Lab-on-a-Chip devices (LOCs), which are devices that integrate one or several laboratory functions on a single chip of only millimeters to a few square centimeters in size. LOCs deal with the handling of extremely small fluid volumes down to less than pico liters. While considerable development has been directed toward producing electrochemical microsensors, such as ion specific or enzyme specific electrodes, little progress has been made in the development of a suitably robust microfabricated reference electrode for use with MEMS or LOC devices. The unavailability of a microfabricated liquid junction reference electrode that is both stable and long-lived has restricted the use of microelectrochemical sensors in industrial and biomedical applications. Heretofore, researchers have used chloridized silver wire as the (pseudo) reference electrode, upon considering the analyte pH and chloride ion concentrations to be relatively constant. However, use of chloridized silver wire renders the sensors non-robust and subject to fouling and failure or unexpected chemistries that change the activity of the potential-controlling compound or ion. In many cases, macroscopic double liquid junction reference electrodes have been used in the absence of a reliable microfabricated reference electrode. However, use of a macroscopic reference electrode makes the sensor as a system large and complex. Therefore, a need exists for an improved microfabricated liquid junction reference electrode.
Accordingly, there is still a need in the art for a nominally co-planar, micro-fabricated liquid junction reference electrode that is highly stable and reliable, robust, and which can be integrated to microfabricated chemical sensors and MEMS devices.